This invention relates to conformable and die-cuttable biaxially oriented films and labelstocks, and more particularly to biaxially stretch-oriented monolayer and multilayer films and labelstocks.
It has long been known to manufacture and distribute pressure-sensitive adhesive stock for labels by providing a layer of face or facestock material for the label or sign backed by a layer of pressure-sensitive adhesive which in turn is covered by a release liner or carrier. The liner or carrier protects the adhesive during shipment and storage and allows for efficient handling of an array of individual labels after the labels are die-cut and the matrix is stripped from the layer of facestock material and up to the point where the individual labels are dispensed in sequence on a labeling line. During the time from die-cutting to dispensing, the liner or carrier remains uncut and may be rolled and unrolled for storage, transit and deployment of the array of individual labels carried thereon.
Failure to reliably dispense is typically characterized by the label following the carrier around a peel plate without dispensing or xe2x80x9cstanding-offxe2x80x9d from the carrier for application to the substrate. Such failure to dispense is believed to be associated with excessive release values between the label facestock material and the liner. Dispensability also is dependent upon the stiffness of the facestock. Failure to dispense may also be characterized by the wrinkling of the label due to lack of label stiffness at the dispensing speed as it is transferred from the carrier to the substrate. Another particular need in many labeling applications is the ability to apply polymeric-film labels at high line speeds, since an increase in line speed has obvious cost saving advantages.
In many label applications, it is desirable that the facestock material be a film of polymeric material which can provide properties lacking in paper, such as clarity, durability, strength, water-resistance, abrasion-resistance, gloss and other properties. Historically, polymeric facestock material of thicknesses greater than about 3 mils (75 microns) have been used in order to assure dispensability in automatic labeling apparatuses. For example, plasticized polyvinyl chloride films about 3.5 to 4.0 mils (87.5 to 100 microns) thick were used in label application because these films exhibited the desired flexibility characteristics. However, the migration of the plasticizers used in PVC films to convert the normally rigid films to flexible films was recognized as a major problem area for these types of films resulting in loss of desirable properties such as adhesion, color buildup, shrinkage, and flexibility. Eventually, migration of the plasticizer results in wrinkling, cracking and visual deterioration of the facestock and/or label. Also, it is desirable to reduce the thickness or xe2x80x9cdown-gaugexe2x80x9d the facestock material in order to attain savings in material costs. Such reduction in facestock thickness often has resulted in reduced stiffness and the inability to die-cut and dispense the labels in a reliable commercially acceptable manner using automatic machinery. There also was pressure for environmental reasons to prepare labels from polymer facestocks other than polyvinyl chloride.
Polymeric materials suggested in the prior art as useful in preparing labels include biaxially-oriented polypropylene (xe2x80x9cBOPPxe2x80x9d) of thicknesses down to about 2.0 mils (50 microns). These materials provide cost savings as they are relatively inexpensive, and they have sufficient stiffness to dispense well. However, these materials also have relatively high tensile modulus values in both machine-direction (MD) and cross direction (CD) which results in labels which are not very conformable.
Related conformability problems have been encountered in respect to rigid surfaces such as glass. For example, when biaxially-oriented films are applied to rigid substrates such as glass bottles, the application is not completely successful. The relatively stiff labels have a tendency to bridge surface depressions and the mold seams resulting from bottle-forming processes resulting in an undesirable surface appearance of the applied label simulating trapped air bubbles. This has somewhat impeded the use of pressure-sensitive adhesive labels to replace prior glass bottle labeling techniques such as ceramic ink directly bonded to the bottle surface during glass bottle manufacturing processes as customers find the appearance unattractive. Such ceramic ink techniques are environmentally undesirable due to objectionable ink components and the contamination of the ink in the crushed bottle glass in recycling processes. Attempts to use the relatively stiff oriented polypropylene films on flexible substrates such as plastic bottles also have not been completely successful because the labels do not have the flexibility required to conform to the flexible plastic containers. Oriented polypropylene films are also more difficult to print than PVC or polyethylene films.
Other useful materials are unoriented polyethylene and polypropylene films that are also relatively inexpensive and conformable. However, both of these films are difficult to die-cut and do not dispense well at low calipers. In Europe, an unoriented, relatively thick polyethylene facestock has been used successfully in preparing labels. The facestock is die-cuttable and the labels can be dispensed in high speed automatic dispensing equipment. The normal thickness of this xe2x80x9cstandardxe2x80x9d polyethylene facestock in Europe is about 4.0 mils (100 microns). Attempts to reduce the gauge of the polyethylene facestock to reduce costs has not yet met with any degree of success because the thinner polyethylene facestock is not readily die-cuttable with the die leaving a mark on the liner and stringers on the cut label. Additionally, the thinner facestock becomes difficult to dispense at higher speeds over a peel plate because of reduced stiffness.
One embodiment of this invention is a die-cuttable, biaxially stretch-oriented monolayer film comprising a polyethylene having a density of about 0.940 g/cm3 or less, a propylene polymer or copolymer, or mixtures thereof, wherein the tensile modulus of the film in the machine direction is greater than the tensile modulus in the cross direction, the tensile modulus of the film in the cross direction is about 150,000 psi or less, and the film is free of copolymers of ethylene with an ethylenically unsaturated carboxylic acid or ester. In one embodiment, the biaxially oriented monolayer films have been biaxially stretch-oriented and heat set.
In another embodiment, the invention relates to a die-cuttable, stretch-oriented multilayer film comprising
(A) a base layer having an upper surface and a lower surface, and comprising polyethylene having a density of about 0.940 g/cm3 or less, a propylene homopolymer or copolymer, or mixtures thereof wherein the base layer is free of copolymers of ethylene with an ethylenically unsaturated carboxylic acid or ester, and
(B) a first skin layer of a thermoplastic polymer bonded to the upper surface of the base layer, wherein the tensile modulus of the multilayer film in the machine direction is greater than the tensile modulus in the cross direction, and the tensile modulus in the cross direction is about 150,000 psi or less. The biaxially oriented multilayer films are useful in particular in preparing adhesive containing labelstock for use in adhesive labels.
In yet another embodiment, the invention relates to a die-cuttable, biaxially stretch-oriented monolayer film comprising at least one polyolefin wherein the film has been stretch-oriented in the machine direction at a stretch ratio of about 9:1 to about 10:1, and in the cross direction at a stretch ratio of from greater than 1:1 to about 3:1.